1. Field of the Invention
This invention relates generally to ovens and, in particular, to a combined microwave and convection oven for use in the domestic or institutional food service industries and to a method of using such an oven.
2. Discussion of the Prior Art
Microwave ovens produce quick, penetrating heat in the center of food objects, beneath their outer surface. This differs from conventional heat that usually takes much longer to penetrate into the product from outside through its outer surface. Accordingly, domestic microwave ovens are commonly used for the very rapid reheating and/or cooking and/or thawing of foods.
Common applications of domestic microwave ovens are:
A. Reheating of ready-made meals that are: PA1 1. Frozen and kept in the freezer at about -18.degree. C. PA1 2. Chilled and kept in the refrigerator at about +4.degree. C. PA1 3. "Shelf-stable", i.e., are sterile and can be kept on the kitchen shelf (usually about +20.degree. C.) without refrigeration. PA1 B. Thawing cum cooking/baking and/or reheating of bakery products such as croissant and pizza. PA1 C. Cooking or popping of popcorn. Microwave popcorn has one major setback, which is that a larger than usual proportion of the corns (called "non-poppers") do not pop. Millions of dollars have gone into research of this problem and no solution has been found. PA1 D. Thawing of frozen meat to be cooked.
A major disadvantage of microwave heating is that it does not brown and dehydrate the exterior of the food as with conventionally heated ovens. With microwaves alone meat cannot be roasted and bread cannot be baked, for instance. Products such as pizza and croissant do not achieve the required crispness and baked appearance, aroma and texture that are their essential organoleptic characteristics. This is true even if they have been fully baked before freezing (should they be frozen).
The combination oven was developed in an effort to overcome the aforesaid major disadvantage.
Such combination ovens combine microwave with conventional heating, usually by using conventional electrical resistance heaters, although also by using gas combustion. Even in such combination ovens, however, a ready-made crisp crust or flaky pastry usually becomes soft, soggy, wet and wrinkled. This is due to the much faster heating by microwaves relative to conventional heating. The come-up time required for the heating elements to become hot and transfer heat to air in the oven (usually to 160.degree. to 250.degree. C.) takes about 10 to 15 minutes. This is much longer than the time required to heat the food with microwaves, which is typically 0.5 to 4.0 minutes.
It is well known that, in microwave heating, the product being heated acts as a heating element converting the electromagnetic waves transmitted directly into heat within its center. Such rapid internal heat generation serves to drive steam from the product center to the colder product surface where condensation occurs rendering the product surface wet.
Combination ovens can usually operate in three basic modes: microwave heating only, convection heating only or the combination of the two modes at the same time.
For operation in the combined mode the total consumption has to be limited by the manufacturer to the maximum power available at the site, commonly between 2000 and 3000 watts.
Another advantage of gas combustion heating over electrical heating is that a much larger quantity of heat can be very rapidly generated, without overloading the domestic mains electricity supply. In addition to that, the microwave energy output could remain simultaneously at its maximum. The simultaneous power consumption by conventional resistance heating elements and the microwave generator has to be balanced so as not to exceed the power limit of a domestic socket. This increases the potential applicability of such known combination ovens that use gas.
Despite the above-mentioned advantages, combination gas-heated ovens do not enjoy great commercial success, compared to those with electrical heating, as they are much more complex and expensive since they require two, independent heating systems of gas and electricity. A gas combustion system of itself is much more complex and cumbersome than an electrical heating system. The microwave generating and transmitting system is inherently extremely complex and expensive, and consequently additional heating systems combined into the oven should, preferably, be as simple and as cheap as possible.
Another problem with gas systems is that a ventilation system is required to expel exhaust gases from the area in which the oven is used. In an institutional environment such ventilation systems should be common-place but in domestic applications this would usually be absent in a domestic kitchen and would add greatly to the overall cost of purchase and installation of the oven system.
An exposed gas combustion system cannot burn simultaneously with microwaves being generated in the same chamber as damage to the gas burner will occur. One known solution to this problem is to install the gas burners within a separate chamber. A shielding partition is placed between the cooking chamber and the burner chamber. The partition is perforated to allow the hot combustion gases through small holes to the cooking chamber, aided by a fan. The holes are sized such that microwaves cannot pass through the partition from the cooking chamber to the burners. This method provides for slower cooking as the flow of heating gases to the cooking chamber is inhibited by the perforated wall. In this case the conventional heat cooking speed would be hindered by the partition from matching that of the microwaves.